Optical disks are widely used as storage media. A CD-ROM optical disk is typically manufactured in accordance with published standards and has prerecorded information. A problem with CD-ROM optical disks is that it is often desired to limit the number of copies that can be made. For example, after the disk is used, the user may copy the information recorded on the disk to a second disk and give the second disk to another user. The disk manufacturer often wants to restrict this type of usage.
One method for disk copy protection uses a hardware key or a special data disk referred to as a key disk for the protection. However, that method requires an additional medium for carrying the key in the form of additional hardware or a separate disk. In addition, that method can make usage of the recording medium impossible if the hardware key or the key disk breaks, or the key disk is misplaced.
U.S. Pat. No. 5,572,589 discloses an alternative technique for optical disk copy protection. That technique requires serializing disks by physically damaging a portion of the optical disk to create a pattern of damage. The pattern of damage is encoded in an identifying value, and the disk is marked with the identifying value to distinguish an authorized optical disk from an unauthorized disk. A drawback to that technique is that it requires sacrificing disk capacity by damaging useful recording area.
Another approach to provide disk copy protection is to record a copy protection code on an optical disk so the code cannot be scanned by an optical head, and to use this code as a key to encrypt the user data before encoding. For example, U.S. Pat. No. 5,274,623 discloses an optical disk having information recorded in a recording area on a top surface of the disk. Copy protection information is recorded on the outer peripheral edge of the optical disk in the form of a serrated pattern perpendicular to the surface of the recording layer.
A further technique to provide disk copy protection is the use of a disk identification number, preferably in the form of a bar code. This technique is disclosed in commonly-assigned U.S. Pat. No. 5,430,281. In addition, commonly-assigned U.S. Pat. No. 5,489,768 discloses an optical disk having bar codes used to prevent storing inappropriate files on the disk by preventing an unauthorized system to interact with the disk.
A problem with previous approaches and techniques is that copy protection information on an optical disk does not fully prevent unauthorized copying of data from the optical disk. Users can often subvert the unauthorized copy feature and make unauthorized copies of the optical disk.
Accordingly, it is an object of the present invention to provide an optical recording medium, such as an optical disk, that can prevent unauthorized reading or copying of data stored on the medium.